Continuous casting

ABSTRACT

A crucible (11) for use in a continuous casting process using upwards feed induced by gas pressure within a surrounding casing (10) consists of a monolithic or one-piece crucible body (28) of refractory material defining a melt chamber (29) having a charging opening (30) at one end and a melt outlet (31) at the opposite end, the melt outlet communicating with a die feed port (35) by way of ducts (32, 34) extending through the crucible body. The melt chamber and the ducts are formed in the body by machine-boring or drilling. The melt outlet (31) also communicates with a narrowed drain port (33B) near which are cooling probe recesses (38) for receiving cooling probes (44) to effect a drain plug of &#34;frozen&#34; melt. The plug is cleared to drain the crucible completely simply by withdrawing the cooling probes.

This invention relates to continuous casting.

The invention is especially but not exclusively concerned withpressurised upward-feed continuous casting.

The invention relates, in particular, to continuous casting apparatus,and to a method of recovering melt from such apparatus.

In continuous casting, solidified metal is drawn from a die which iscontinuously fed with melt. The draw can be either vertical orhorizontal; and the feed can be either upwards or downwards. Thesepossibilities are already known. Where upwards feed is required, it hashitherto been necessary by means of suction or pressure to induce meltto enter and flow in a nozzle or tube of which a lower end is immersedin the melt and the upper end is connected to feed the die. The melt isheld in a crucible. With these known arrangements for upward feed,various disadvantages are experienced. For example, it is virtuallyimpossible to induce all of the melt into the feed tube; and theremaining melt can be recovered from the crucible sometimes only withdifficulty subsequent to a casting "run". Moreover, effecting lastingand leak-free connection between the feed tube and the die is difficult.Also, changing a die is not feasible without bringing the entireoperation to a relatively low temperature.

An object of the present invention is to provide improvements incontinuous casting apparatus and method whereby one or more of theaforesaid disadvantages is/are obviated or mitigated.

According to one aspect of the present invention, there is providedcontinuous casting apparatus comprising a casing, a crucible within thecasing, means for sustaining a raised gas pressure within the casing sothat the pressure acts in and around the crucible, and feed duct meansfor carrying melt from a melt chamber in the crucible to a casting dieleading from the casing; characterised in that the crucible has aone-piece body of a refractory material, and the feed duct means extendsthrough the body of the crucible from a relatively lower inlet in themelt chamber to a relatively upper outlet at a feed port formed in thebody of the crucible.

According to another aspect of the present invention, there is provideda method of recovering melt from a continuous casting apparatus; whereinthe apparatus comprises a crucible having a one-piece body of arefractory material with an integral drain port, and said methodcomprises the steps of locally cooling the vicinity of the drain portwhilst producing melt in the crucible so that a plug solidifies in thedrain port, and withdrawing or stopping said cooling so that heat fromthe melt liquifies said plug.

According to yet another aspect of the present invention, there isprovided a crucible comprising a one-piece body of a refractorymaterial, and a melt chamber in said body with a charging opening at anupper end of the chamber; characterised in that the chamber has a meltoutlet from a lower end of the chamber, first duct means extends throughthe body from said melt outlet to a drain port, feeding duct meansextends through the body from the first duct means to an upper feed portformed in said body, and the body is recessed in the vicinity of thedrain port to receive or accommodate means for effecting localisedcooling of said vicinity.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic sectional elevation of a continuous castingfurnace incorporating a crucible in accordance with the presentinvention;

FIG. 2 is a view in the direction of arrow A in FIG. 1 of part of thecrucible seen in FIG. 1;

FIG. 3 is a view on arrow B in FIG. 1 of a lower part of the crucibleseen in FIG. 1;

FIG. 4 is an enlarged sectional elevation of part of the crucible seenin FIG. 1; and

FIG. 5 is a sectional elevation of a modified crucible, similar to partof FIG. 1.

In the drawings, a continuous casting apparatus has an outer casingindicated generally by reference numeral 10, a crucible 11 within thecasing 10 and capable of being brought to a working temperature by meansof a heating element 12, a continuous casting die 13, and withdrawingrolls 14.

More particularly, the casing 10 consists of a generally rectangularsteel shell 15 and a lid or cover 16 capable of being secured to theshell 15 in co-operation with a seal 17 by means of fixing bolts (notshown). A centre portion of the lid 16 is provided with a verticaltubular housing 18 arranged to accommodate the upper portion of thecasting die 13. The top fact of the die 13 is engaged by a further seal19 and located by an annular cover 20.

The cover 16 is fitted with a duct 21 providing a means for supplyingthe interior of the casing 10 with a gas under pressure. Also, the duct21 may be used for the purpose of evacuating the interior of the casing10. The cover 16 is also fitted with an "air-lock" charging port 22 bymeans of which charge metal may be introduced to the interior of thecasing 10 under operating conditions. Finally, the cover 16 is fittedwith a gas discharge and pressure control valve 23. The valve 23features a needle and metering orifice at 24, discharge ports 25, andweights 26 which can be removed or added to adjust the gas dischargepressure.

The casing 10 is lined with a thermal insulating material 27.

The crucible 11 consists of a one-piece rectangular body 28 of arefractory material such as graphite. The body 28 is machined to definea melt chamber 29 having a charging opening 30 at the top end and a meltoutlet 31 at the opposite lower end. A first duct means 32, drilledthrough the body 28, extends between the melt outlet 31 and a drain portat 33, the duct 32 following a descending path to assist draining. Asecond duct means 34 extends through the body 28 from an intersectionwith the duct 32 to a feed port at 35. The body 28 is further machinedto define a cylindrical recess 36 which accommodates and supports thelower end of the casting die 13. The recess 36 is concentric with theduct 34, and leak-proof connection between the duct 34 and the castingdie 13 is effected by means of a seal 37 of a flexible refractorysealing material such as GRAFOIL (trade mark).

To each side of the drain port 33, the body 28 is provided with proberecesses 38; and similar probe recesses 39 are provided in the body 28to each side of the die recess 36. The function of these probe recesses38, 39 is explained further herebelow. The body 28 is supported withinthe casing 10 on a refractory tile or block 40. A drainage collectingcrucible 41 is positioned below the body 28 for receiving residue meltas explained hereunder.

Electrical connections 42 for the heating element 12 are routed to theoutside of the casing 10 through a seal at 43.

In line with the probe recesses 38, the casing 10 carries twowater-cooled probes 44 which extend into the casing 10 through a seal 45operable to permit insertion and withdrawal of the probes 44 so that theprobe tips can selectively be disposed within the probe recesses 38.Similarly, in line with the probe recesses 39, the housing 10 carriessix further water-cooled probes 46 which extend into the housing 10through a seal 47 operable to allow insertion and withdrawal of theprobes so that the probe tips may be disposed within the probe recesses39.

The casting die 13 is of known construction, made from a refractorymaterial such as graphite. The die 13 is equipped in known manner withposition-adjustable water-cooled probes 48 operable to ensure that theliquidus/solidus interface during casting is positioned approximately atthe level indicated by arrow C in FIG. 1.

Greater detail of the construction of the drain port 33 of FIG. 1 can beseen in FIG. 4. When the duct 32 has been drilled into the graphite body28, a plug 33A is inserted into the duct and a smaller drain hole 33B ismade through the plug 33A. Operation of the drain port is explained ingreater detail hereunder.

Operation of the continuous casting furnace is as follows. When assemblyof the furnace is complete, metal charge is introduced into the meltchamber 29 by means of admission through the charging port 22. Theprobes 46 are set to a withdrawn position, and the probes 44 areinserted so that their tips are disposed within the probe recesses 38.The heating element 12 is energised and when melt is produced it flowsthrough the melt outlet 31, through duct 32 and into the small drainhole 33B the vicinity of which is cooled. Accordingly, a "frozen" plugof the charge metal forms in the hole 33B so blocking the drain port 33.Continued production of melt results in the liquid metal ascendingwithin duct 34 until levels equalise in this duct and in the meltchamber 29. A starting wire is positioned within the casting die 13 inknown manner. The continuous casting process is started by increasingthe pressure of inert gas flowing into the casing 10 through the duct21, the increase in pressure acting on top of the metal in melt chamber29 being sufficient to raise the level in duct 34 to enable continuouscasting.

The casting die 13 may be replaced without first allowing the entirefurnace to cool and without damage to the body 28 by oxidation. Thus,prior to opening the die housing 18 the probes 46 are inserted into theprobe recesses 39 thereby to cool the vicinity of the body 28 at theinterface between the casting die and the feed port 35, bringing thetemperature in this vicinity to about 400° C. Then, the casting die maybe replaced, the probes 46 withdrawn, working temperature regained andthe casting can continue.

On completion of a casting run and/or when all of the melt has beenused, the probes 44 are withdrawn so that the vicinity of the drain port33 heats sufficiently to melt the metal in the hole 33B. Thereafter,metal remaining in the body 28 drains into the collecting crucible 41.This arrangement enables maximum economy which is especially significantwhen costly or precious metals are being used.

The crucible shown in FIG. 5 incorporates modifications within the scopeof the present invention. In FIG. 5, features corresponding with thosealready described with reference to FIG. 1 are given the same referencenumerals as used in FIG. 1. First, in FIG. 5, the crucible body 28features an additional cylindrical recess 50 which is intended toaccommodate a casting die (not shown) from which metal can be drawn inthe horizontal mode of continuous casting. It will be understood thatthe shell 15 and associated equipment would be appropriately modified.The die recess 50 is supplied by means of a feed port 51 whichcommunicates with the duct 34 by means of a short branch duct 52 whichis inclined to assist drainage of molten metal therefrom. It isenvisaged that both vertical and horizontal modes of continuous castingmay be performed simultaneously. Alternatively, either of these modes isperformed whilst the feed port or casting die of the other is closed bymeans of a graphite or other ceramic plug attached to a starter bar (notshown). Vertical mode continuous casting is particularly suited to theproduction of tubular material, whereas horizontal mode continuouscasting is more appropriate for the production of long rods, etc.

FIG. 5 also incorporates a modification to the drain plug arrangements.Thus, the relatively simple inserted plug 33A of FIG. 4 is replaced by agraphite plug 33C which is larger than the plug 33A and has asignificant portion of its mass disposed further away from the cruciblebody 28. The plug 33C features a narrower duct portion 33D whichcommunicates with a still narrower drain hole 33E. Probe recesses 38Aextend into the plug 33C on either side and in the vicinity of the drainhole 33E. The drain plug arrangement shown in FIG. 5 operates in thesame manner as described above, but with reduced thermal effect on themelt in ducts 32 and 34 whilst the cooling probes are positioned withinthe probe recesses 38A to effect the "frozen" plug.

The crucible configurations described above may be used with metalswhich are aggressive to graphite by first lining the melt chamber, theducts 32, 34 and the casting bore of the die 13 with a suitablerefractory ceramic. This lining can be accomplished by using hard liningelements and/or by means of coating with a ceramic slurry.

During a casting process, the gas pressure within the casing 10 is heldat a constant pressure throughout the casting cycle at a value more thansufficient to raise liquid metal in the duct 34 to the liquidus/solidusinterface. Such pressure would generally be between 0.2 and 0.8 bardepending upon the liquid density of the metal or alloy being cast.

It will be noted that by means of the drain port arrangements, completerecovery of all melt can be achieved at any time. This is a particularlyuseful facility when working with ceramic or refractory lined crucibles.

It is envisaged that continuous casting furnaces as above described maybe constructed on virtually any scale. However, dimensions typical of aprototype furnace found to work satisfactorily are as follows. The body28, of graphite, is approximately 25 cm square as viewed in plan, andapproximately 40 cm high. The melt chamber 29 is about 6 cm diameter,and each of the ducts 32, 34 is about 10 mm diameter.

In modifications of the casting process and/or apparatus described,within the scope of the present invention, the crucible body is providedwith multiple melt chambers; one or more melt chambers is/are connectedto feed two or more casting dies; and the drain port arrangements usingthe "freeze/re-melt" plugging technique are used in an otherwiseconventional crucible. Also, it is envisaged that the drain port(particularly of FIG. 5) may be cooled to effect "freeze" plugging bycooling means not featuring probe recesses; for example a cooling jet ofgas may be used, directed at the plug body.

I claim:
 1. Continuous casting apparatus comprising a casing 10, a meltchamber (29) in a crucible (11), the crucible (11) placed within thecasing (10), a charging opening (30) at an upper end of the melt chamber(29), means for sustaining a raised gas pressure within the casing (10)so that the pressure acts in and around the crucible (11), the feed ductmeans (32,34) for carrying melt from the melt chamber (29) to a castingdie (13) leading from the casing (10); the crucible being made as aone-piece body (28) of a refractory material, and the feed duct means(32,34) extending through the body (28) of the crucible (11) from arelatively lower inlet (31) in the melt chamber (29) to a relativelyupper outlet at a feed port (35) formed in the body (28) of the crucible(11).
 2. Apparatus as claimed in claim 1 wherein part of said duct means(34) branches to serve multiple feed ports (35, 51).
 3. Apparatus asclaimed in claim 2 wherein said multiple feed ports include a feed port(35) disposed to feed melt for vertical drawing, and a feed port (51)disposed to feed melt for horizontal drawing.
 4. Apparatus as claimed inclaim 1 wherein part of said duct means (32) communicates with a drainport (33) for discharge of all melt from the crucible (11).
 5. Apparatusas claimed in claim 1 wherein the melt chamber (29) and the duct means(32, 34, 51) are formed by machine bores or drill holes extending insaid body (28).
 6. Apparatus as claimed in claim 4 wherein the ductmeans (32) narrows or is reduced in size in the vicinity of the drainport (33).
 7. Apparatus as claimed in claim 4 wherein the body (28) inthe vicinity of the drain port (33) is recessed (38) to receive oraccommodate means for effecting localized cooling of said vicinity. 8.Apparatus as claimed in claim 6 wherein a plug (33A, 33C) is insertedinto the drain port (33), said plug defining narrowing or reduced-sizeducts (33B, 33D).
 9. Apparatus as claimed in claim 2 wherein the body(28) at each feed port (35, 51) defines a recess (36, 50) for receivinga continuous casting die (13), and there is provided a seal (37) in therecess (36, 50) for engagement by the die (13).
 10. Apparatus as claimedin claim 9 wherein the body (28) is recessed (39) in the vicinity of thefeed port (35) to receive or accommodate means for effecting localisedcooling of the body (28) in the vicinity of the feed port.
 11. Apparatusas claimed in claim 1, wherein the casing (10) is provided with acharging port (22) for introducing charge metal to the interior of thecasing (10), and the charging opening (30) of the crucible (11) isdisposed below said charging port (22).
 12. A crucible comprising aone-piece body (28) of a refractory material, a melt chamber (29) insaid body (28) with a charging opening (30) at an upper end of the meltchamber (29), a melt outlet (31) at a lower end of the melt chamber(29), first duct means (32) extending through the body (28) from saidmelt outlet (31) to a drain port (33), feeding duct (34) extendingthrough the body (28) from the first duct means (32) to an upper feedport (35, 51) formed in said body (28), and a recess (38) formed in thebody (28) in the vicinity of the drain port (33), said recess (38)serving to receive or accommodate means for effecting localized coolingof said vicinity.